1. Field of the Invention
The present invention relates to a method of fabricating implanted wells and islands of CMOS circuits in silicon based large-scale-integrated complementary insulated-gate field-effect transistor circuits. The islands are of the substrate conductivity type with the wells having the opposite conductivity type.
2. Description of the Prior Art
With the conventional, standard CMOS process, the well takes up a relatively large amount of space. To increase the packing density, the depth of the well must be reduced. This, however, would increase the vertical current gain of parasitic bipolar transistors and, thus, increase the risk of latch-up.
One solution is the retrograde-well process described in the journal "IEEE Trans. Electron. Dev.", October 1981, pp. 115 to 119. "Retrograde wells" are understood to include wells and islands with a shallower doping profile and reduced lateral diffusion in which the dopant concentration first increases with increasing depth and then decreases from a given point, unlike the situation in conventional wells where the dopant concentration decreases continuously with increasing depth. In this manner, the dopant concentration at the surface can be reduced to a low level as is needed by the MOS transistor, but the overall concentration can be kept high enough that the current gain of parasitic bipolar transistors will be sufficiently small.
In the process for forming retrograde wells, unlike in the conventional CMOS process, the oxidation at the edge region of the well, i.e., the field oxidation, is carried out first, and only then are the ions implanted, which is followed by a conventional brief annealing treatment.
If the conventional process is to be used to fabricate twin wells, i.e., two wells lying side by side and containing at least one p-channel transistor and at least one n-channel transistor, respectively, a second photomasking step using an inverse well mask is necessary. The doping of the field regions is then dependent on the fieldoxide thickness, the bevel of the field-oxide edge, and the well implantation. If the doping is to be controlled independently of these parameters, two additional masks are necessary for the field oxidation. This leads to increased spacings due to alignment tolerances.